Research on the coefficient of sound absorption in turbid water

J. Marine. Sci, Appl. (2008) 7:135-138DOI: 10.007/511804-008-7036-6Research on the coefficient of sound absorption inturbid waterLIU Yong-wei" (刘永伟) , LIQi (李琪)College of Underwater Acoustic Engineering, Harbin Engineering University, Harbin 150001, ChinaAbstract: China's coastal waters are turbid and the properties of the seabed are cormplex. This negativelyimpacts the performance of underwater detection equipment. The properties of sound absorption inturbid water are not well understood. In this paper, the coefficient of sound absorption in turbid waterwas measured by the reverberation technique. All work was done in a reverberation barrel made ofseamless aluminum. First, pure water was poured into the reverberation barrel and its reverberation timemeasured. Next, various concentrations of turbid water were poured into the barrel and theirreverberation time measured. After all data had been gathered, the coefficient of sound absorption inturbid water of different concentrations was calculated. From this we determined a law of soundabsorption in turbid water as summarized in the paper.Keywords: reverberation time; sound absorption; turbid waterCLC number: TB566Document code: AArticle ID: 1671-9433(2008)01-0135-041 Introductionhave begun to do research on measuring properties ofsound absorption in water medium. The work hasThe water of our coastal ocean is extraordinary turbid.mainly been done on pure water and electrolyteTo compare the turbid sea water and clear sea water,solutions. There are several methods to do thesound absorption in turbid water is increasing irresearch, such as plane wave method, resonanceevidence and sound scattering is enhancingmethod and reverberation method. Ref.[1-6]remarkably. It is positive that the capability of usualdemonstratedthat the research was mainly done onunderwater acoustic detection equipments decreases inpure water and electrolyte solutions at differentturbid sea water. In order to increase the capability ofconcentrations in the past. But the research on theunderwater acoustic detection equipments in turbidturbid sea water was relatively less. These methodssea water, an efficient way is to decrease the workingcan also be used for studying properties of turbid seafrequency. Because sound absorption in medium haswater.linear dependence on squared frequency, the relationbetween the intensity of sound scattering and2 Basic principlefrequency is complicated. The intensity of soundscattering may be caused by scattering objects' kinds,The theory of reverberation had been brought forwarddimensions and distribution etc. It is not simple 0by Saibin (W.C.Sabine) in 1900. Sound sourceeasy to decrease the working frequency of theradiates energy and gets to steady state in the soundunderwater acoustic detection equipments. In order todiffusing field. Then the sound source stops working.maintain the power and direction identically, theThe time taken for the sound pressure level to fll bydimension of transducers must be enlarged and the60dB is calld reverberation time. It can be signed byperformance of space resolution must be sacrificedToo usually.with frequency decreased. The decreasing extent ofthe frequency depends on the cognition of propertiesThe中国煤化工。caculation is givenof sound absorption and sound scattering in turbid seabyTYHCNMHGThese data are nearly blank either55.26Vinternationally or domestically. Since 1940s, scientistsc(aS +8βV)°(1)Reverberation time eflects the speed of sound energyReceived date: 2007-05-16.*Correspengtirs 理thor Email: liuyongwei000@sohu.com136 .J. Marine. Sci. Appl. (2008) 7:135-138attenuation in a reverberation room (barrel). It alsoresults stable and credible.reflects the condition of sound absorption in aThe reverberation barrel was cylindrical in thereverberation room (barrel) indirectly. The more thexperiment. It was an aluminum ingot forged. Thesound is absorbed, the quicker the sound energybarrel had no brazing seam. Its inner diameter wasattenuates and the shorter the reverberation time is. So100 cm. Its height was 54 cm. Its wall thickness wasthe relation between reverberation time and sound4.5 cm. The minimum frequency measured was aboutabsorption can be utilized. If reverberation time is3 kHz. The aluminum barrel's inner surface had beenmeasured, the coefficient of sound absorption will becovered by lacquer in order to prevent the barrelcalculated.surface from corrosion. The aluminum barrel wassupported at four points of its lower rim by means ofThe basic principle is as follows: when thehard-wood chocks in order to reduce sound radiation.reverberation barrel is flled up with pure water orThe diagrams of the experimental system connectionturbid water, the energy loss of the container surfaceand practical experimental apparatus are shown inand measuring system is identical, so pure water canFigs.1~3.be used to calibrate total energy loss of thePULSEreverberation barrel's inner surface and measuringsystem. The mean coefficient of sound absorption ofthe equivalent reverberation barrel's inner surface can| Measatrg Anpbe calculated by Sabin formula. If the reverberationtime of pure water and turbid water is measuredRervrberaton barrespectively, the coefficient of sound absorption inturbid water can be calculated by the formula. TheIlyuophonecalculation formula is given by(6060ThanducerB=Tc Toco+B,(2)Fig.1 Scheme of the experimental system's connectionwhere B。is the coefficient of sound absorption inpure water, P。 is the coefficient of sound absorptionin turbid water, T。is the reverberation time of purewater, T is the reverberation time of turbid water, Cand c are the sound speed of pure water and turbidwater respectively.β。and C。 can be looked up in the handbook. Ref.[7]demonstrates that the speed discrepancy betweenFig.2 Design of degassing and dampingturbid water and normal sea water is very small. ccan be considered as the speed of sea water.3 Experimental designsWithin low frequency range, the coefficient of soundabsorption is very small. The coefficient of sound中国煤化工absorption in sea water is at 10-3 level. When theexperimental system is designed, the coefficient offYHCNMHGsound absorption caused by reverberation barrel mustbe small. The system also needs to be damped so as toFig.3 Practical scheme of experimental systemreduce the sound energy loss through radiation. Themediymred needs to be degassed to ensure theThere are several methods for measuring theJ. Marine. Sci. Appl. (2008) 7:135-138137reverberation time, such as steady random noisecalculation with formulas looked up in the handbooks,cut-off method, impulse backward integration method,the measuring error of the measuring system wasetc. The first method was adopted to measure theobtained.reverberation time in the experiment. The signalsource in the PULSE (type B&K 3560E) transmitted4 Experimental resultsthe random noise. Then, the signal was magnified bythe power amplifier (type B&K 2713). The magnifiedThe results of the comparison between the datasignal was loaded to the transducer. A steady soundmeasured and the data of calculation with formulasfield would be built up in the reverberation barrel.looked up in the handbooks are shown in Fig.5.After the signal source was cut off, the hydrophone0.020(type B&K8103) watched the decay of sound pressure0.018。+ 0.2% therylevel. The signal received by the hydrophonc was0.0161-0134% eperiena0.014magnified by measuring amplifier (type B&K 2692).0.012The magnified signal was collected by PULSE. The口0.010decay of sound pressure level was recorded by0.0080.006PULSE. The reverberation time was calculated by the0.004decay of sound pressure level to fall by 60 dB, which0.002had been done by the reverberation time template in) 15PULSE.Frequency/kHz170Fig.5 Coefficients of sound absorption in sulfate magnesium令1600.050，1015221500.0450.040j-075%4410省1400.035130百0.0300.025120可0.020a 1100.0150.0101000.00524Reverberation time/s10 15 20 25.. 3035 40Fig.4 Decay curve in the experimentFig.6 Coefficients of sound absorption in turbid waterThe frequencies recorded were 12.5, 16, 20, 25, 31.5,40 kHz in the experiment. In order to verify theThe solute in turbid water was magnesium sulphate andmeasurement error, the aluminum barrel was flled upthin soil. The mass percentage of magnesium sulphatewith pure water. After degassing, the reverberationwas 0.34%. It was equal to 0.014 mol/L. The coefficienttime of pure water was recorded. The reverberationof sound absorption in the magnesium sulphate solutiontime was measured many times and the data recordedof which the concentration is 0.014 mol/L is equal to thewas averaged, so as to reduce the random measuringcoefficient of sound absorption in sea water. Theerror. After looked up the coefficient of soundmaking process of thin soil was as follows: flower soilabsorption and sound speed of pure water in thewas bought from the market. Then the flower soil washandbook, the loss of sound energy through radiationput into the plastic barrel. (The proportion was thatand the change caused by temperature were all treatedlkg flower soil needs 20 L water to be dissolved).as the coefficient of sound absorption caused by theAfter mixed round sufficiently, the branches floatingaluminum barrel. After that, pure water was changedand leaves rotting in the water were filtered away. Therest中国煤化工o the plastic barrel.by magnesium sulphate solutionat differentconcentrations. After degassing, their reverberationThe 1|YHC N M H Gottom of the plasticbarrel were tnrown away. Atter tne rest of the solutiontime was recorded.The coefficient of soundabsorption in magnesium sulphate solution at differentin plastic barrel had been deposited for a period (48concentrations was calculated through Eq.(2). Byhours usually), the thin sludge at the bottom of thecomparison, between the data measured and the data ofplastic barrel was bailed out and dried out. The dried138J. Marine. Sci. Appl. (2008) 7:135-138sludge had been crushed by a hammer. After theis much bigger than that in magnesium sulphate.crushed sludge had been griddled by clinical gauze(5~6 layers), the thin soil was put into the preparedFrom Fig.6, it is also shown that the cofficient of soundmagnesium sulphate solution. The solute in the turbidabsorption becomes decline with frequencies at somewater was thin soil, and its mass percentage was 0.15,concentrations. This is because the resonator frequency0.3，0.5, 0.75 respectively. The data measured haof the mud grain in turbid water is about 25 kHz. So thebeen processed by Matlab. Fig.6 shows the results.cofficient of sound absorption in turbid water is biggerThe standard deviation of the data was less than 0.10at 25 kHz than the one at other frequencies.equally. The standard deviation has no unit. It onlydemonstrates the fluctuation of the data. That theReferencesstandard deviation of the data was less than 0.1equally showed that the fluctuation of the data was[1] Chinese Physics Comitte. Underwater acoustics[M].Beijing: Science press, 1960: 58-66, 98(in Chinese).small and the data was reliable.2] GARRISON G R, FRANCOIS R E, PENCE E A. Soundabsorption measurements at 10-60 kHz in near-freezing5 Experimental conclusionssea water[]. The Journal of the Acoustical Society ofAmerica, 1975, 58(3): 608-619.Fig.5 shows the discrepancy between the experiment[3] FISHER F H, SIMMONS V P. Sound absorption in seaand calculation by theory exists. First, even ifwater[J]. The Journal of the Acoustical Society ofdegassing is done in the reverberation barrel, thereAmerica, 1977, 62(3): 558-564.still exists some bubble in the water. It makes the[4] SCHNIDER H G, THIELE R, WILLE C P. Measurementmeasured coefficient of the sound absorption irof sound absorption in low salinity water of the Balticmagnesium sulphate greater. Second, the loss of soundSea[J]. The Journal of the Acoustical Society of America,energy through radiation is bigger comparatively1985, 77(4): 1409-1412.within low frequency ranges; however, the experiment[5] BROWN N R, TIMOTHY G, RICHARDS L, et al.Measurement of viscous sound absorption at 50-150kHzhas been done within this frequency ranges. Thein a model turbid environment[J]. The Journal of thecoefficient of sound absorption is much smaller, so itAcoustical Society of America, 1998, 104(4): 21 14-2120.is difficult to do the experiment. If the work is done in[6] QIU Xinfang. A cylindrical resonator method for thehigh frequencies, the experiment will be fulfilledinvestigation of low-frequency sound absorption in seaeasily due to small radiation. The experimental resultswill also be reliable. Third, the calibration of theAmerica, 1991, 90(6): 23-63.system's measuring error is compared between the [7] SCHULKIN M, MARSH W H. The sound absorption ofexperimental data and calculation with formula. Thesea water[J] The Journal of the Acoustical Society ofcalculated data is not the material data. This alsoAmerica, 1962, 34(6): 864-865.makes that the experimental results do not accord with[8] WANG Rongjun. Underwater acoustic materialsdirectory[J]. Beijing: Science press, 1983:114-115(inthe calculated data. However, from Fig.5， theChinese).maximum experimental data is two times than thecalculated data or so. This error can be ignored, if theLIU Yong-wel, borm in 1981, is a doctor atlaw of sound absorption in turbid water is studied. TheHarbin Engineering University. His currentresearch is focused on the proprties of turbidlaw of the sound absorption in magnesium sulphate atscawater.different concentrations from the experiment is thesame as that calculated with formula.From Fig.6, three aspects can be shown as follows:1) The coefficient of sound absorption in turbid water中国煤化工s8, isa pressr at HartinCisit, and the Dean of theis augmenting with the concentration augmenting.MHC N M H Ger Acoustice Eieering His2) At a fixed concentration, the coefficient of soundcurent research includes properties of turbidabsorption is nearly augmenting with frequencyseawater, etc.augmenting.3) The cofficient of sound absorption in turbid water